Background: Primary percutaneous coronary intervention (PCI) is the treatment of choice for patients presenting with ST-elevation myocardial infarction (STEMI). In patients with STEMI, QRS prolongation, in the absence of bundle branch blocks, was considered mainly a dynamic phenomenon likely to be improved by successful reperfusion. Aim:  To evaluate the role of QRS duration change following primary angioplasty in patients presented with STEMI as a marker of microvascular reperfusion assessed by myocardial blush grade (MBG). Methods:  Fifty patients with STEMI were included with this study, 37 males and 13 females, their age ranged between 32 to 74 years. ECG was done to all patients at presentation and after primary PCI to asses QRS duration change .Angiographic assessment of myocardium supplied by infarct related artery was performed using MBG. According to MBG patients were classified into two groups: group (1), with good myocardial reperfusion MBG (2-3) and group (2), with poor myocardial reperfusion MBG (0-1). Results:  Although there is no difference between two groups according to sex, smoking, hypertension and diabetes, patients with higher age were at risk of poor tissue reperfusion (p=0.049). Patients with poor tissue reperfusion had longer QRS duration on presentation and after PCI compared to normal perfusion group (P<0.001).There was significant positive correlation between QRS narrowing and left ventricular ejection fraction, on the other hand there was significant negative correlation between QRS narrowing and door to balloon time . Conclusion:QRS duration change post angioplasty is a strong marker of myocardial reperfusion in patients presenting with STEMI.